1. Field of the Invention
This invention relates to a method and device for storing and generating of ammonia from storage materials capable of binding and releasing ammonia reversibly. In particular, the storage materials are solid metal ammine complexes capable of binding and releasing ammonia reversibly. The method and device may be used in the selective catalytic reduction of NOx.
Other applications using ammonia in mobile or portable units or in special chemical synthesis routes where storage of liquid ammonia is too hazardous are also contemplated embodiments of the present invention. This also includes fuel cell systems where ammonia may be considered an efficient hydrogen carrier.
2. Description of the Related Art
As disclosed in applicant's co-pending application (WO2006012903) metal ammine salts can be used as a solid storage media for ammonia which in turn may be used as the reductant in selective catalytic reduction to reduce NOx emissions from automotive vehicles, boilers and furnaces. Thus, the metal-ammine salt constitutes a solid storage medium for ammonia, which represent a safe and practical option for storage and transportation of ammonia. This is advantageous compared with NOx removal using ammonia delivered as aqueous solution of urea or ammonia because the large fraction—typically more than 65%—of water is avoided. In particular, Mg(NH3)Cl2 represents an ammonia storage material characterized by a high degree of safety because the vapor pressure at room temperature is below 0.1 bar. It is also characterized by a high mass density of ammonia because Mg is a light metal. Applicant's co-pending application (WO2006081824) discloses further storage materials with a high, demonstrated volumetric capacity and method of making them.
International Patent Publication No. WO 99/01205 discloses a method and a device for selective catalytic NOx reduction in waste gases containing oxygen, using ammonia and a reduction catalyst. According to the method, gaseous ammonia is provided by heating a solid storage medium preferably being granulated material consisting of Ca(NH3)8Cl2 or Sr(NH3)8Cl2.
However, the use of the ammonia storage media known from WO 99/01205 suffers from various draw-backs hampering a wide-spread use in the automotive industry. In order to release ammonia, the material has to be heated. This is not a problem by itself, but the material is rather volatile and in the case that the control of the heating fails one can reach a situation of over-heating and thus reach very high ammonia pressures in a storage container—even at temperatures below 100° C.
The use of Mg(NH3)6Cl2 is much safer and the heated unit has to reach temperature above 100° C. in order to get an ammonia desorption pressure above 1 bar. Thus, having a desorption unit consisting of MgCl2 is an excellent combination of thermal desorption and safety. However, Mg(NH3)Cl2 releases ammonia in three steps. First, four ammonia molecules are released in a temperature range between 80-200° C. The last two molecules (per unit MgCl2) are released at temperatures above 200° C. and these two will require more energy to desorb because the desorption temperature is higher and the enthalpy of desorption is higher than that for the first four molecules. Therefore, a compact storage system that operates only with ammonia release in the temperature range of 80-200° C. has an advantage in terms of safety. However, a container with a more volatile storage material (e.g. Ca(NH3)8Cl2 or Sr(NH3)8Cl2) that does not involve heating to such high temperatures would be commercially interesting because the container could be made of a polymer due to the fact that the temperature used is below 100° C.
Other ammonia adsorbing/absorbing materials have similar problems in that they are either safe, since they have a low ammonia pressure, but require comparatively much energy for ammonia desorption, or require less energy for ammonia desorption, but are less safe, because high ammonia pressures may be built up.
Thus there is a need for a new technology that combines the safety of thermal desorption of MgCl2 or another ammonia adsorbing/absorbing material having a low ammonia pressure without having the challenge of heating very large quantities of storage material above e.g. 200° C. in order to get all ammonia released from the storage unit.